This invention relates to a novel process for metallizing ceramic surfaces, as with a conductor pattern, in such a way as to be able to form reliable seals between a conductive metallic pattern to be developed on a ceramic material and metal leads therefrom.
There are a number of problems associated with the ceramic-metallizing processes, and the ceramic to metal bonding process, of the prior art. Generally, the processes require (a) use of noble metals or (b) use of a hydrogen atmosphere for processing of relatively base materials like nickel. The noble metals are extremely expensive, but the use of nickel and hydrogen presents hazards which require expensive safety precautions. Moreover, nickel and other such metals tend to form oxides on storage which are not readily wetted by solder and which interfere with subsequent soldering. Also, some of the non-noble metals such as copper tend to migrate excessively ("plate out") during processing, storage or use.
Also, it is to be recognized that noble metals provide real problems when utilized in manufacture of components intended for high voltage applications. Silver conductors, so used, often have some silver metal migrate completely out of the primary conductor path in the form of discrete metal or in the form of whiskers. Similar problems are faced with such metals as gold, cadmium, and like metals which have little bonding force between atoms within the metal lattice. Thus, it would be desirable to provide a solderable conductive coating which does not require such metals. As mentioned above, base metals like nickel have required hydrogen firing, to achieve the desired sintering, once painted on a ceramic surface. Even then, they usually must be soldered immediately, or carefully stored in a non-oxidizing atmosphere, to avoid the formation of oxides which will interfere with the reliable formation of a good bond between the metallized coating on the ceramic and the conductor leading from the ceramic. Indeed, presence of such oxides usually requires the use of acid fluxes which are considered to be inconvenient and otherwise undesirable for many electronic applications.
Another problem encountered in present methods of metallizing ceramic surfaces is the so-called "solder-leaching" problem. This relates to a phenomena wherein silver in a lead is leached therefrom when the lead is dipped into a solder pot. Normally, this effect is minimized by adding about 2% to 5% silver to the solder; but, even with this expedient, the properties of silver conductors remain sensitive to dwell time within the solder pot.
It is already known in the art to provide metallizing paints for use on ceramic substrates such as alumina, silica, and the like. For example, U.S. Pat. No. 3,590,468 to Buck discloses a metallizing paint composition utilizing a glass frit, nitrocellulose and molybdenum trioxide or manganese dioxide to use in forming alumina-to-metal seals: The paint is first sintered and reduced to form a metal bearing coating; this process takes place at 1000.degree. C. to 1500.degree. C. in a wet hydrogen atmosphere. Apparently, a molybdenum or manganese film is formed during reduction. Normally more than one coat is applied to the ceramic substrate. Similar art is disclosed in U.S. Pat. No. 3,429,736 to Thompson et al; U.S. Pat. No. 3,347,704 to Meyer et al; and U.S. Pat. No. 3,403,043 to Thompson et al.
More recently, Patterson in U.S. Pat. No. 3,943,168, discloses particulate compositions which are formed of a special composition which is a combination of nickel borides and nickel silicides. This material, which is a solid solution, did not provide adequate oxidation resistance during firing. Some oxidation of Ni.sub.3 B to B.sub.2 O.sub.3 started as early in the firing cycle as 325.degree. C. This drawback is recognized in subsequently-issued U.S. Pat. No. 4,064,310 to the same inventor.
In view of the increasing cost of using noble metals, in view of the unsuitability of these metals for many applications, (such as high voltage applications), and in view of the relatively expensive and inconvenient alternatives to their use, Applicant has turned his attention to providing improved processes and products which make entirely practical the metallizing of ceramic surfaces without resorting to use of noble metals and which enable one to minimize the use of special storage conditions and to avoid use of hydrogen gas in metallizing with non-noble metals.